This invention relates generally to the preparation of trimers, tetramers, and polymers of ethylene catalyzed by derivatives of certain metal complexes.
The selective trimerization of ethylene to prepare primarily 1-hexene, and ultimately to form polymers therefrom, has been extensively studied and a number of catalysts developed. Examples include the well known chromium pyrrolide complexes, disclosed in U.S. Pat. Nos. 5,523,507, 5,786,431, and elsewhere; trialkylsilylamide-chromium (II) complexes on activated inorganic refractory compounds in combination with aluminum triallyl compounds, disclosed in U.S. Pat. No. 5,104,841; chromium diphosphines, disclosed in Chem. Comm. (2002) p 858; chromium cyclopentadieniyl catalysts as disclosed in Angew. Chem. Int. Ed. 38 (1999), p 428, J. Poly. Sci., 10 (1972), p 2621, and Applied Catalysis A; General 255, (2003), p 355-359; silica supported trialkylsilylamide-chromium complexes in combination with isobutylalumoxane, disclosed in J. Mol. Cat. A: Chemical, 187, (2002), p 135-141; mixed heteroatomic compounds disclosed in Chem, Comm. (2003), p 334; tantalum compounds disclosed in Angew. Chem. Int. Ed., 42, (2003), p 808-810; titanium cyclopentadiene catalysts such as those of Angew. Chem, Int. Ed., 40, (2001), p 2516; and numerous others. In U.S. Pat. No. 5,137,994, a process for producing ethylene/1-hexene copolymers directly from ethylene using silica supported chromium compounds was disclosed. Control of polymer density was obtained by adjusting the ethylene/1-hexene ratio of the intermediate monomer mixture obtained in an initial trimerization.
Oligomerization and polymerization of higher olefins such as propylene and 1-butene is disclosed in U.S. Pat. No. 4,668,838. The general mechanism of trimerization is considered to involve metalloheptane ring formation and most likely agnostic assisted hydride transfer, as disclosed in Angew. Chem. Int. Ed., 42, (2003), 808-810. The foregoing processes are highly useful for the selective formation of trimers in the substantial absence of higher oligomer or polymer formation. There remains a need for the discovery of processes for the selective formation of tetramers, especially 1-octene from ethylene.
Stepwise ethylene chain growth on aluminum alkyls was discovered in the 1950's by K. Ziegler et al. The reaction is normally conducted at temperatures in the range of 100°-200° C. under high ethylene pressure, typically 2000-4000 psi (14-28 MPa). At higher temperatures, a displacement reaction or cracking step competes with chain growth, producing α-olefins and regenerating aluminum alkyl compounds. For a review see, “Comprehensive Organometallic Chemistry: 1982, Pergammon Press, Vol. 7, Section 46. The process may be advanced by catalysts, both for the step-wise growth of the aluminum alkyl and the catalyzed displacement of α-olefins therefrom. Ziegler-Natta catalysts such as those discovered by Kaminsky et al. Angew. Chem. Int. Ed. Engl., 1976, Vol. 15, pages 630-632 may be used to catalyze the growth process. This process is thought to involve active transition metal catalysts which promote the growth of the aluminum alkyl chains. Chain growth is terminated in the displacement or cracking step, principally by β-hydrogen- or β-alkyl-elimination to give a vinylic end group or by hydrogenolysis to give a paraffinic end group, thereby regenerating a catalytically active transition metal-hydride or alkyl and an aluminum-hydride or alkyl.
The manufacture of α-olefins using the foregoing step addition to aluminum alkyls is commercially practiced in large volume. Suitable processes operating at lower temperatures and pressures than those employed by early artisans are disclosed in U.S. Pat. No. 5,276,220 (using actinide metal metallocene based complexes, which unfortunately are radioactive, as catalysts) and in U.S. Pat. No. 5,210,338 (using metallocene based complexes of zirconium and hafnium).
A persistent problem of the foregoing step growth processes is the production of aluminum alkyls and the resulting α-olefin products having a narrow molecular weight distribution (Poisson distribution). In many such processes the products are undesirably broad, and best described by the Schulz-Flory statistical distribution. These statistical distributions are commonly known and defined by the equations: Poisson: Xp=(xp·e−x)/p!, and Schulz-Flory: Xp=β(1+β)−p, where Xp is the mole fraction with p added monomer units, x is the Poisson distribution coefficient equal to the average number of monomers added, and β is the Schulz-Flory distribution coefficient. A typical Schultz-Flory distribution of α-olefins would provide a maximum of 15 percent 1-hexene and 17 percent 1-octene. Additionally, significant quantities of 1-decene and higher α-olefins (C12-30 α-olefins) are produced, along with low molecular weight waxy polymers.
Despite the advance in the art encompassed by the foregoing known processes, a process that operates at milder temperatures and pressures to produce primarily trimers (1-hexene) and tetramers (1-octene), especially in a Poisson type distribution, while limiting less valuable butene, C10-30 α-olefin, and low molecular weight polymer formation is still desired. The α-olefin products of the present process, especially 1-hexene and 1-octene are useful industrial chemicals employed to prepare alcohols and plastics, especially high molecular weight, linear low density, polyethylene.
Further desired is a process for preparing high molecular weight polymers and/or copolymers of ethylene and branch inducing α-olefins such as 1-hexene and/or 1-octene (linear low density polyethylene) directly from ethylene.